Electrohydraulic torque transfer device and control system

ABSTRACT

A power transmission device includes a rotatable input member, a rotatable output member, a friction clutch to selectively transfer torque between the input member and the output member, an actuator providing an actuating force to the friction clutch and a controller. The actuator includes an electric motor having an output shaft drivingly coupled to a gerotor. The gerotor is operable to supply pressurized fluid to a piston acting on the friction clutch. The controller controls the actuator in response to a four-wheel lock request to provide one of a minimum output torque and an output torque greater than a vehicle requested torque to operate the friction clutch in a locked mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/201,468 filed on Aug. 11, 2005. The disclosure of the aboveapplication is incorporated herein by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to a power transmission deviceoperable to selectively transfer torque between first and second sets ofdrivable wheels of a vehicle. More particularly, the present inventionis directed to a power transmission device adapted for use in motorvehicle driveline applications having an actuator including an electricmotor drivably coupled to a gerotor for providing pressurized fluid to apiston acting on a friction clutch.

Due to increased demand for four-wheel drive vehicles, many powertransmission systems are typically being incorporated into vehicledriveline applications for transferring drive torque to the wheels. Manyvehicles include a power transmission device operably installed betweenthe primary and secondary drivelines. Such power transmission devicesare typically equipped with a torque transfer mechanism for selectivelytransferring drive torque from the primary driveline to the secondarydriveline to establish a four-wheel drive mode of operation. At leastone known torque transfer mechanism includes a dog-type lock-up clutchthat may be selectively engaged for rigidly coupling the secondarydriveline to the primary driveline when the vehicle is operated infour-wheel drive mode. Drive torque is delivered only to the primarydriveline when the lock-up clutch is released and the vehicle operatesin a two-wheel drive mode.

Another type of power transmission device is operable for automaticallydirecting drive torque to the secondary wheels without any input oraction on the part of a vehicle operator. When traction is lost at theprimary wheels, four-wheel drive mode is engaged. Some transfer casesare equipped with an electrically-controlled clutch actuator operable toregulate the amount of drive torque transferred to a secondary outputshaft as a function of changes in vehicle operating characteristics suchas vehicle speed, throttle position and steering angle. Typically in thepower transfer device is a clutch positioned within the transfer casehousing.

While many power transfer devices are currently used in four-wheel drivevehicles, a need exists to advance the technology and recognize thesystem limitations. For example, the size, weight and packagingrequirements of the power transmission device may make such system costsprohibitive in some four-wheel drive applications.

The present invention provides a power transmission device including afriction clutch operable to selectively transfer torque between an inputmember and an output member. An actuator is operable to provide anactuating force to the friction clutch. The actuator includes anelectric motor having an output shaft drivingly coupled to a gerotor.The gerotor is operable to provide pressurized fluid to a piston actingon the friction clutch. The gerotor substantially dead-heads and theoutput shaft of the electric motor rotates at approximately 600 rpmduring actuation of the friction clutch. However, the electric motorrotation speed may vary based on gerotor size, clearances betweengerotor and gerotor housing, and the operating pressure.

In one embodiment, the power transmission device includes a controlleroperable to determine a magnitude of torque to be transferred. Thecontroller controls the actuator to pressurize fluid within a closedcavity containing a piston acting on a friction clutch to generate therequested magnitude of torque. The controller is operable to vary thesupply of electrical energy to the motor via pulse width modulation tovary the output of a positive displacement pump and vary the outputtorque of the friction clutch. The motor is operable to continuouslyrotate while torque is being transferred by the friction clutch.

A power transmission device may include a rotatable input member, arotatable output member, a friction clutch to selectively transfertorque between the input member and the output member, an actuatorproviding an actuating force to the friction clutch and a controller.The actuator includes an electric motor having an output shaft drivinglycoupled to a gerotor. The gerotor is operable to supply pressurizedfluid to a piston acting on the friction clutch. The controller controlsthe actuator in response to a four-wheel lock request to provide one ofa minimum output torque and an output torque greater than a vehiclerequested torque to operate the friction clutch in a locked mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic of a four-wheel drive vehicle equipped with apower transmission device of the present invention;

FIG. 2 is an exploded perspective view of an exemplary powertransmission device;

FIG. 3 is a cross-sectional side view of the power transmission deviceof FIG. 2;

FIG. 4 is another cross-sectional side view of the power transmissiondevice of FIG. 2;

FIG. 5 is a schematic depicting the components of a torque transfersystem including the power transmission device of the presentdisclosure; and

FIG. 6 is a flow chart relating to a four-wheel lock mode of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

The present invention is directed to a power transmission device thatmay be adaptively controlled for modulating the torque transferredbetween a rotatable input member and a rotatable output member. Thetorque transfer mechanism may be useful within motor vehicle drivelinesas a stand-alone device that may be easily incorporated between sectionsof propeller shafts, directly coupled to a driving axle assembly, orother in-line torque coupling applications. Accordingly, while thepresent invention is hereinafter described in association with aspecific structural embodiment for use in a driveline application, itshould be understood that the arrangement shown and described is merelyintended to illustrate an exemplary embodiment of the present invention.

With reference to FIG. 1 of the drawings, a drive train 10 for afour-wheel vehicle is shown. Drive train 10 includes a first axleassembly 12, a second axle assembly 14 and a power transmission 16 fordelivering drive torque to the axle assemblies. In the particulararrangement shown, first axle 12 is the front driveline while secondaxle 14 is the rear driveline. Power transmission 16 includes an engine18 and a multi-speed transmission 20 having an integrated frontdifferential unit 22 for driving front wheels 24 via axle shafts 26. Atransfer unit 28 is also driven by transmission 20 for delivering torqueto an input member 29 of a coupling 30 via a driveshaft 32. The inputmember 29 of the coupling 30 is coupled to driveshaft 32 while itsoutput member is coupled to a drive component of a rear differential 36.Second axle assembly 14 also includes a pair of rear wheels 38 connectedto rear differential 36 via rear axle shafts 40.

Drive train 10 is shown to include an electronically-controlled powertransfer system 42 including coupling 30. Power transfer system 42 isoperable to selectively provide drive torque in a two-wheel drive modeor a four-wheel drive mode. In the two-wheel drive mode, torque is nottransferred via coupling 30. Accordingly, 100% of the drive torquedelivered by transmission 20 is provided to front wheels 24. In thefour-wheel drive mode, power is transferred through coupling 30 tosupply torque to rear wheels 38. The power transfer system 42 furtherincludes a controller 50 in communication with vehicle sensors 52 fordetecting dynamic and operational characteristics of the motor vehicle.The controller is operable to control actuation of coupling 30 inresponse to signals from vehicle sensors 52. The controller 50 may beprogrammed with a predetermined target torque split between the firstand second sets of wheels. Alternatively, the controller may function todetermine the desired torque to be transferred through coupling 30 viaother methods. Regardless of the method used for determining themagnitude of torque to transfer, controller 50 operates coupling 30 tomaintain the desired torque magnitude.

FIGS. 2-4 depict coupling 30 in greater detail. Coupling 30 includes aninput shaft 70 selectively drivingly coupled to an output shaft 72 via afriction clutch 74. A drive flange 75 is mounted on one end of inputshaft 70 to provide a mounting provision for a driveline component suchas driveshaft 32.

Coupling 30 includes a substantially cup-shaped housing 76 having asubstantially cylindrically-shaped side wall 78 and an end wall 80. Sidewall 78 includes an internally threaded portion 81 near the open end ofhousing 76. An end cap 82 is threadably engaged with threaded portion 81to define a cavity 84. Alternatively, end cap 82 may be fastened to thehousing using other techniques including spaced apart threadedfasteners. End cap 82 includes an aperture 86 extending therethrough. Aportion of output shaft 72 extends through aperture 86. Housing 76includes an aperture 88 extending through end wall 80. A portion ofinput shaft 70 extends through aperture 88. Bearings 90 are positionedwithin aperture 88 to rotatably support input shaft 70. Bearings 91 and92 rotatably support an output spindle 93. Input shaft 70 includes asplined portion 95 (FIG. 2) drivingly coupled to a hub 94. A set ofinner friction plates 96 are drivingly coupled to hub 94 via a splinedengagement. Inner friction plates 96 are interleaved with a plurality ofouter friction plates 98. Outer friction plates 98 are in splinedengagement with a drum 100. Drum 100 is drivingly coupled to outputspindle 93. Output spindle 93 is coupled with output shaft 72 viaanother splined interface. In the embodiment depicted, friction clutch74 is a wet clutch. Accordingly, clutch fluid is contained within cavity84 in communication with friction plates 96 and 98.

A piston 104 is slidably positioned within a cavity 106 formed withinhousing 76. Piston 104 is axially moveable into engagement with a thrustbearing 108 and an apply plate 110. When pressurized fluid acts on aface 112 of piston 104, piston 104 translates and applies a forcethrough thrust bearing 108 and apply plate 110 to the plurality ofinterleaved clutch plates 96 and 98. Torque is transferred between inputshaft 70 and output shaft 72 via the components previously describedwhen friction plates 96 and 98 are forced into contact with one another.

An actuator 120 is mounted to housing 76 to selectively supplypressurized fluid to cavity 106 and provide an apply force to frictionclutch 74. Actuator 120 includes an electric motor 122, a pump 124, anda reservoir 126. Electric motor 122 includes an output shaft 127drivingly engaged with pump 124 such that rotation of the output shaftof the electric motor causes fluid within reservoir 126 to bepressurized and enter cavity 106. A bleed screw 128 is coupled tohousing 76 in communication with cavity 106. Bleed screw 128 functionsto allow an operator to purge trapped air from the closed hydraulicsystem. This minimizes the power required to compress trapped air.

Pump 124 includes a housing having a first half 130, a second half 132and a gerotor 134. Gerotor 134 includes an inner gear 136 and an outerrotor 138 in engagement with one another. Inner gear 136 is drivinglycoupled to the output shaft of electric motor 122. In operation, lowpressure fluid passes through an inlet port 140 formed in housing half130. Inlet port 140 is in fluid communication with reservoir 126.Rotation of inner gear 136 relative to outer rotor 138 causes a pumpingaction to force highly pressurized fluid through an outlet port 142formed in housing half 130. Outlet port 142 is in fluid communicationwith a passageway 144 formed in pump housing half 130. Passageway 144 ispositioned in fluid communication with an aperture 146 formed in housing76. In this manner, fluid output from gerotor 134 is supplied to cavity106 to act on piston 104.

One skilled in the art should appreciate that gerotor 134 acts on aclosed volume of fluid located within passageway 144 and cavity 106.Because gerotor acts on the closed volume of fluid, electric motor 122rotates at a relatively high rpm for only a relatively short amount oftime when the clearance between piston 104, thrust bearing 108, applyplate 110 and the interleaved friction plates 96 and 98 is eliminated.After the clearance has been taken up, piston 104 transfers force toapply plate 110 to cause friction clutch 74 to generate torque. At thistime, piston 104 does not axially move and gerotor 134 enters a neardead-head mode. Due to the existence of a clearance between inner gear136 and outer rotor 138 of gerotor 134, as well as a clearance betweengerotor 134 and the pump housing, the output shaft of electric motor 122continues to rotate inner gear 136 at a relatively low rotational speeddependent on gerotor size, clearances and pressure to maintain a desiredpressure acting on piston 104. Some of the fluid trapped withinpassageway 144 and cavity 106 passes by inner gear 136 and outer rotor138 in the reverse direction thereby allowing the output shaft of theelectric motor to continue to rotate. If the gerotor were completelysealed and did not allow any backflow or blow by, the electric motorwould be forced to stop due to the incompressible nature of the fluidbeing pumped by gerotor 134.

As shown in FIG. 5, controller 50 is in communication with electricmotor 122 as well as a pressure transducer 150. Pressure transducer 150is operable to output a signal indicative of the fluid pressure withincavity 106. Controller 50 operates using a closed-loop feedback controlto actuate electric motor 122 to maintain a target pressure acting onpiston 104. Controller 50 is operable to provide a pulse width modulatedsignal to electric motor 122 to vary the output speed of the motor andthe output pressure generated by pump 124. The pressure within cavity106 should be proportional to the magnitude of torque output by frictionclutch 74. By controlling the pressure maintained within cavity 106, thetorque transferred through coupling 30 is controlled. Furthermore, atemperature sensor 152 is coupled to coupling 30 and is operable toprovide controller 50 a signal indicative of the temperature of theclutch fluid contained within cavity 84. The controller 50 is programmedto vary the coupling control strategy based on clutch fluid temperature.The control strategy attempts to protect the clutch fluid fromoverheating.

In an alternate embodiment, a pressure relief valve 200 (FIGS. 4 and 5)is plumbed in communication with the high pressure passageway 144.Pressure relief valve 200 is operable to allow pressurized fluid to passfrom the high pressure side of pump 124 to the low pressure side atreservoir 126. Pressure relief valve 200 provides a path for the fluidwithin the previously described closed volume to escape. When pressurerelief valve 200 allows flow therethrough, electric motor 122 may beoperated at a higher rotational speed than previously described in thenear dead-head operational mode of the pump. Depending on the type ofelectric motor fitted to coupling 30, it may be more or less desirableto incorporate pressure relief valve 200 into coupling 30. Specifically,if the electric motor may be operated at relatively low rotationalspeeds between 0-100 rpm for extended duration, it may not be necessaryto include a pressure relief valve. On the contrary, if an electricmotor design is chosen than must operate at higher rotational speeds, itmay be desirable to include the pressure relief valve in order toprovide a flow path for the fluid. It should also be appreciated thatany number of gear arrangements may be inserted between the output shaftof electric motor 122 and the inner gear 136 of gerotor 134 therebyallowing the motor to operate a higher rotational speed while rotatingthe pump components at a low rotational speed. If a speed reducinggearset is used, a pressure relief valve is not necessarily required.Similarly, a pressure relief valve may be alleviated by optimizing thepump size, pump and housing clearances and the operating pressure.

FIG. 6 provides a control logic flow diagram for an optional four-wheeldrive lock mode of operation. A control system 300 for the four-wheellock feature begins at a vehicle ignition step 302. Once vehicleignition is on, control proceeds to step 304 where the four-wheel lockmode is inactivated.

A standard torque transmitting device diagnostic and control logic isimplemented at step 306. Control proceeds to a decision block 308 wherecontrol determines whether a four-wheel lock mode has been requested.Four-wheel lock mode may be requested by any number of means such as auser operated switch preferably located on the dashboard or otherwisenear the vehicle operator. Additionally, a signal requesting four-wheellock mode may be transmitted by the vehicle's bus from other systemssuch as a stability control system.

If a four-wheel lock mode has not been requested, control returns toblock 306 where standard torque transmitting device diagnostic andcontrol logic is implemented. If it is determined that a four-wheel lockmode has been requested, control continues to a decision block 310.Control determines whether a vehicle speed is within a predeterminedrange and whether a steering input is within a predetermined range aswell. If both the vehicle speed and steering criteria are not met,control returns to block 306. If the vehicle speed and steering criteriahave been met, control continues to a decision block 312. Block 312determines whether a temperature of the clutch fluid temperaturecontained within cavity 84 is less than a predetermined value. Thetemperature of the clutch fluid may be indicated by a signal output fromtemperature sensor 152 or calculated using a temperature model. If thetemperature of the clutch fluid is greater than or equal to thepredetermined value, control returns to block 306. If the clutch fluidtemperature is less than the predetermined temperature, controlcontinues to a decision block 314.

Block 314 determines whether a vehicle requested torque is less than thetorque transmission device minimum torque. Vehicle requested torque maybe determined by monitoring the throttle input from the vehicle driver.The torque transmission device minimum torque may be set by determiningthe amount of pressure the pump motor may sustain for extended periodsof operation without exceeding its operational temperature limit. Thisminimum torque value may be adjusted according to ambient conditions andvehicle operating conditions. For example, the torque transmissiondevice minimum torque may be decreased if ambient air temperature isgreater than a threshold. Furthermore, the preset minimum torquetransmission device torque may also be decreased if the vehicle supplyvoltage is low. Other operating conditions may be evaluated to vary thetorque transmission device minimum torque.

If block 314 determines that the vehicle requested torque is less thanor equal to the torque transmission device minimum torque, controlcontinues to a decision block 316. Decision block 316 determines whetherdiagnostic shut down conditions are present. If a diagnostic system orsome other vehicle system indicates that four-wheel lock mode should notbe entered, it is considered to be a diagnostic shut down. Accordingly,control returns to block 306 where standard torque transmission devicediagnostic and control logic is implemented.

If a diagnostic shut down signal is not present, control proceeds toblock 318 where transfer device torque is set to the torque transferdevice minimum torque. Control continues to step 320 where pump 124 iscontrolled to provide the minimum torque transfer device torque. Forexample, coupling 30 may be operated by providing a current ofapproximately 3 amps to electric motor 122. Electric motor 122 drivespump 124 to output pressurized fluid from outlet port 142 atapproximately 50 psi. Fifty psi acts on piston 104 to compress theclutch plates of friction clutch 74. Based on this arrangement, aminimum torque will be transferred through coupling 30. For themagnitude of current and pressure developed within the examplepreviously described, it is estimated that 500 Nm of torque will beproduced by coupling 30. It should be appreciated that the magnitude oftorque, pressure developed and current provided are merely examples.Other values may be used depending on the particular characteristics ofthe motor, pump and clutch being used. In one example, the torquetransfer device minimum torque corresponds to the torque that may bemaintained for an extended period of time operating coupling 30 at anambient temperature of 70° F. without forced air flow. Once the pump isoperating to provide the minimum torque transfer device torque, controlreturns to block 306.

Returning to decision block 314, if vehicle requested torque is greaterthan the torque transfer device minimum torque, control continues toblock 322. Block 322 sets the torque transfer device torque to thevehicle requested torque multiplied by a lock factor. Block 322 isstructured in this manner to minimize the time at which friction clutch74 of coupling 30 may be less than fully locked. More particularly, itis desirable to minimize the heat generated by the coupling 30 duringoperation. Less heat is generated if friction clutch 74 operates in afully locked mode as opposed to a mode where the interleaved clutchplates 96 and 98 of friction clutch 74 engage one another but also sliprelative to one another. Accordingly, when a vehicle torque isrequested, it is desirable to set the maximum torque transferable bycoupling 30 to a value slightly higher than the vehicle requested torqueto allow friction clutch 74 to operate in a locked mode.

By way of example, if the driver throttle input equates to a torquerequest of 700 Nm, control will increase current to motor 122 to providea torque transfer device torque equal to 700 Nm times an exemplary lockfactor of 1.1 equaling 770 Nm. Once the torque transfer device torquehas been calculated at step 322, control continues to decision block324.

Block 324 determines if diagnostic shut downs are present. In a mannersubstantially similar to the decisions made within block 316, block 324returns control to block 306 if diagnostic shut downs are present. Ifdiagnostic shut down signals are not present, control continues to astep 326 where pump 124 is operated to provide the torque transferdevice torque equaling the vehicle requested torque times the lockfactor. Once pump 124 is activated, control returns to block 306. If avehicle requested torque exceeds the torque capacity of coupling 30, themaximum torque of coupling 30 will be provided but the interleavedplates 96,96 may slip relative to one another.

As an optional method to minimize motor heating, data may be collectedrelating to the wheel accelerations of the rear axle. If the wheelaccelerations of the rear axle exceed the wheel acceleration produced bythe vehicle's rear wheel skid torque, then the torque transfer devicetorque may be reduced to provide a torque that is slightly greater thanthe skid torque of the surface. For example, the torque transfer devicetorque may return to the torque transfer device minimum torque if thewheel accelerations have the characteristics of being on ice.

As another optional feature of the four-wheel lock system previouslydescribed, a lamp visible by the vehicle operator may be provided toindicate activity or inactivity of the four-wheel lock system. Inparticular, if the four-wheel lock mode has been requested butactivation has been disabled or denied, then a dash lamp may beilluminated to indicate that four-wheel lock mode has not been entered.Also, a dash lamp may be continuously illuminated while the four-wheellock mode is being provided.

Furthermore, the foregoing discussion discloses and describes merelyexemplary embodiments of the present invention. One skilled in the artwill readily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationsmay be made therein without department from the spirit and scope of theinvention as defined in the following claims.

1. A method of controlling a torque transfer device providing four-wheeldrive, the method comprising: determining a temperature of a fluidwithin the torque transfer device; determining whether a vehiclerequested torque is greater than a minimum torque transfer device outputtorque; and controlling the torque transfer device to output one of saidminimum torque transfer device output torque and a torque greater thansaid vehicle requested torque to minimize slip between clutch plates ofsaid torque transfer device when operating in a fully locked mode. 2.The method of claim 1 further including placing said torque transferdevice in said fully locked mode when a steering input angle is within apredetermined range.
 3. The method of claim 1 further including placingsaid torque transfer device in said fully locked mode when a vehiclespeed is within a predetermined range.
 4. The method of claim 1 whereinsaid controller determines a temperature of a fluid within said frictionclutch and controls said actuator based on said temperature.
 5. Themethod of claim 4 wherein said locked mode is not entered when saidfluid temperature is above a predetermined value.
 6. The method of claim1 further including directing pressurized fluid from a pump along asingle fluid line to directly act on a piston to actuate a frictionclutch.
 7. The method of claim 1 wherein said torque greater than saidvehicle requested torque is determined by multiplying said vehiclerequested torque by a lock factor greater than
 1. 8. The method of claim1 wherein the torque transfer device generates a magnitude of torque tomaximize a time that said torque transfer device may operate beforeexceeding a threshold temperature.
 9. The method of claim 8 furtherincluding varying said minimum output torque based on an ambient airtemperature.
 10. The method of claim 8 further including varying saidminimum output torque based on a battery voltage.
 11. A method ofcontrolling a torque transfer device that is configured to transmitrotary power between a first component and a second component, therotary power having a torque with a magnitude, the method comprising:establishing a magnitude of a minimum torque associated with the rotarypower; determining whether a magnitude of a requested torque is greaterthan the magnitude of the minimum torque transfer device output torque;controlling the torque transfer device to output rotary power such thatthe magnitude of the torque of the rotary power is equal to themagnitude of the minimum torque when the magnitude of the minimum torqueis greater than or equal to the magnitude of the requested torque; andcontrolling the torque transfer device to output rotary power such thatthe magnitude of the torque of the rotary power is greater than themagnitude of the requested torque when the magnitude of the minimumtorque is less than the magnitude of the requested torque.
 12. Themethod of claim 11, wherein a predetermined lock factor is employed whenthe magnitude of the minimum torque is less than the magnitude of therequested torque.
 13. The method of claim 12, wherein the magnitude ofthe torque of the rotary power is equal to the product of thepredetermined lock factor and the magnitude of the requested torque. 14.The method of claim 11, further comprising re-establishing the magnitudeof the minimum torque based on one or more predeterminedcharacteristics.
 15. The method of claim 14, wherein the one or morepredetermined characteristics comprises a temperature of a hydraulicfluid employed to control the magnitude of the torque of the rotarypower.
 16. The method of claim 14, wherein the one or more predeterminedcharacteristics comprises an ambient air temperature.
 17. The method ofclaim 14, wherein the one or more predetermined characteristicscomprises a voltage output from a power supply.
 18. The method of claim11, wherein the torque transfer device comprises a friction clutch andwherein controlling the torque transfer device comprises controlling aforce acting on the friction clutch.
 19. The method of claim 18, whereinthe force acting on the friction clutch is controlled via a fluidpressure.